[Octave-bug-tracker] [bug #61845] [octave forge] (image) speed of imrotate with bicubic interpolation increased

[Hartmut]

Follow-up Comment #6, bug #61845 (project octave):

I had a look into what happens in imremap.m that makes imrotate with bicubic
interpolation so slow, now. The profiler shows, that most of the computing
time is spent in the function intpolcub in bicubic_conv in inremap.m. Most of
the time there is due to the two functions cubic12 and cubic01. And these
functions eventually spend most of their time with simple arithmetic
calculations: plus, minus, power of n, times, all with matrices of the size of
the image.

My conculsion now is: It is not easy to speed this up. There seem to be two
ways to do this:
* Find a clever way to do the same calculation in m-code as now, but using
existing functions in core Octave (like corr2) that also calculate correlation
and convolution. Those functions will probably do the job faster, because
their inner part does the calculation in compiled C code. (But I am not sure
if there is a equivalent formulation of the same calculation that uses those
core functions.)
* Rewrite the caculation bit of this code path in a compiled Octave C file.


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